UN Secretary-General Ban Ki-moon warned Friday that any possible airstrikes against Sunni extremists in Iraq could be ineffective and backfire. He urged Iraq’s feuding communities to unite against the terrorists who have captured a vast swath of territory.

A powerful truck bomb exploded on Friday in a government-held village in central Syria, killing at least 34 civilians and wounding more than 50, as an Al-Qaeda breakaway group captured a major town to the east, near the Iraqi border, activists said.

Fighters from the Islamic State of Iraq and Al-Sham stormed the town of Muhassan on the Euphrates river after rebels from the Western-backed Supreme Military Council defected to the jihadi group, activists said. The village is in the eastern oil-rich province of Deir el-Zour where the Islamic State has been on the offensive since late April against rival jihadi and Islamic groups.

The capture of Muhassan, about 100 kilometres from the Iraqi border, and two nearby villages, comes a week after the group swept across wide areas in northern and central Iraq, capturing that country’s second largest city of Mosul, and carving out a large region straddling the border.

“The capture of Muhassan is one of the biggest victories for the Islamic State in Deir el-Zour because it is home of the main centre of the military council in the province,” said Rami Abdurrahman, who heads the Britain-based Syrian Observatory for Human Rights.

The UN chief also urged the Iraqi government and its supporters not to retaliate against Sunni communities in revenge for “barbaric attacks” by the Al-Qaeda-inspired Islamic State of Iraq and Al-Sham.

The Iraqi government has sought U.S. airstrikes to stem the insurgency by Sunni extremists who are pressing toward Baghdad. President Barack Obama has held off for now, but the U.S. is collecting aerial intelligence over Iraq. If the U.S. were to proceed with airstrikes, officials did not rule out hitting targets in neighbouring Syria, where ISIS also has deep ties.

Ban warned that “military strikes against (ISIS) might have little lasting effect, or even be counter-productive, if there is no movement towards inclusive government in Iraq.”

The secretary-general addressed the Iraq crisis in a speech to the Asia Society on Syria, saying “suddenly, the cohesion and integrity of two major countries, not just one, is in question.”

Calling sectarian warfare “a disaster for all,” Ban said it is “imperative” that the Iraqi government and its supporters avoid reprisals against Sunni communities.

“The Sunni extremists of (ISIS) are trying to show that the governments in Baghdad, Iran and the United States are working together to support atrocities against Sunnis,” Ban said. “This would help them mobilize support from the Sunni majority that does not share the extremists’ agenda. It is essential that the government of Iraq and its supporters do everything possible to avoid falling into this trap.”

The secretary-general said arms and fighters have crossed the porous border between Syria and Iraq, and he urged religious and political leaders from the region — including Saudi Arabia, which backs Syria’s rebels, and Iran, which supports Syrian President Bashar Al-Assad’s government — to call for restraint and avoid further sectarian violence.

Addressing the regional threat from extremists was one of six points in the secretary-general’s new blueprint to address the Syrian conflict following the resignation of Lakhdar Brahimi as the joint U.N.-Arab League envoy.

Ban said he intends to appoint a replacement for Brahimi soon and expressed “anger and disappointment” at the growing view that nothing can be done “except to arm the parties and watch the conflict rage.”

Ban said the death toll from the “horrific” war, now in its fourth year, may be well over 150,000, with half the country’s more than 22 million people displaced from their homes.

He demanded international action including a U.N. arms embargo on all parties in the Syrian conflict. He urged individual countries to take action if the Security Council remains deadlocked, which is likely.

“I recognize that an arms embargo at this time would risk freezing an imbalance in place, given the extent of the government’s weaponry,” Ban said.

“But the Syrian war cannot be won by military means. The sides will have to sit across from each other again at the negotiating table. The only question is, how many more people must die before they get there?”

Russia, which is Syria’s closest ally and major arms supplier, has called an arms embargo unenforceable.

“If they explain to us how the arms embargo will be enforced vis-a-vis armed groups, then we’ll consider it,” Russia’s U.N. Ambassador Vitaly Churkin told reporters. “In Libya, when there was an arms embargo, weapons were flowing freely to various opposition groups and are still floating all over Africa, so we do not want to go in that direction.”

Ban also demanded that foreign powers and groups halt military support to the combatants and called on all parties to immediately release detainees _ an idea raised at the failed Geneva political negotiations earlier this year. He asked Russia and China, who vetoed a Security Council resolution that would have referred the Syrian crisis to the International Criminal Court, “to come forward with credible alternatives” to ensure accountability for serious crimes.

Churkin retorted that accountability and reconciliation “must be dealt with by the Syrians as they try to settle this conflict.” He added that Moscow saw the push for ICC referral not as an attempt to get justice but as “another attempt … to put more pressure on the Syrian government.”

“Certainly, you cannot realistically expect that the ICC will go after opposition leaders,” Churkin said, “and going after terrorist leaders, it does not make much sense, because they couldn’t care less.”